This invention relates generally to an electrophotographic printing machine, and more particularly concerns an optical system adapted to attenuate the spectral energy distribution of the light rays being transmitted therethrough.
In the process of electrophotographic printing, a photoconductive surface is uniformly charged and exposed to a light image of the original document. Exposure of the photoconductive surface creates an electrostatic latent image corresponding to the original document. Toner particles are then electrostatically attracted to the latent image to render it viewable. Subsequently, the toner powder image is transferred to a sheet of support material and permanently affixed thereto so as to produce a copy of the original document. The foregoing process is described in detail in U.S. Pat. No. 2,297,691 issued to Carlson in 1942.
Multi-color electrophotographic printing is substantially identical to the heretofore discussed process of black and white printing with the following distinctions. Rather than forming a total light image of the original, the light image is filtered producing a single color light image which is a partial light image of the original. The foregoing single color light image exposes the charged photoconductive surface to create a single color electrostatic latent image. This single color electrostatic latent image is developed with toner particles of a color complementary to the single color light image. The single color toner powder image is transferred from the electrostatic latent image to a sheet of support material. This process is repeated a plurality of cycles with differently colored light images and the respective complementary colored toner particles. Each single color toner powder image is transferred to the sheet of support material in superimposed registration with the prior toner powder image. In this manner, a composite multi-powder image is produced on the sheet of support material. This multi-color powder image is coalesced and permanently affixed to the sheet of support material forming a color copy.
In multi-color electrophotographic printing, the density of the toner particles deposited on the photoconductive surface, i.e. the electrostatic latent image, is dependent upon the voltage level of the development system and that of the electrostatic latent image. Toner particles are attracted to those areas of the photoconductive surface having a voltage thereon greater than that of the development system. The areas of the photoconductive surface irradiated by the single color light are discharged. The degree of discharge is dependent upon the intensity of the light rays impinging thereon. Hence, the photoconductive surface will have a charge gradient thereon, those areas having a charge greater than the voltage level of the development system will attract toner particles thereto, while those areas having a charge less than the voltage level of the development system will not attract toner particles thereto. The voltage level of the development system is selected so that the undesired light rays transmitted from the original document remain undeveloped. This frequently produces a substantial gradient between the voltage level of the development system and the discharge regions of the photoconductive surface increasing the possibility of arcing, toner concentration, carrier bead stickage and trail edge deletion effects. Furthermore, it reduces the total potential gradient available for development. System developability may be appreciably improved by reduction of the voltage level of the development system so as to increase the voltage gradient between those areas of the photoconductive surface having a charge greater than the development system, which decreases the voltage gradient between those areas of the photoconductive surface having a charge less than the development system. However, the voltage level of the development system may only be reduced by eliminating the unwanted low density light rays.
Accordingly, it is a primary object of the present invention to improve the optical system by attenuating the low density light rays transmitted therethrough.